Campbell Scientific SDM-CAN Datalogger to CANbus Interface - Manual (Page 25)

SDM-CAN Datalogger-to-CANbus Interface
The relationship between these times is summarized by:
t
bit
=t
q
+t
TSEG1
+t
TSEG2
t
TSEG1
(in seconds) is set using the scaling factor TSEG1 (parameter 03), the
value of which is calculated using the following equation:
TSEG1 = t
TSEG1
/ t
q
t
TSEG2
is set using scaling factor TSEG2 (parameter 04) the value of which is
calculated using:
TSEG2 = t
TSEG2
/ t
q
When determining the settings of these parameters it is important to ensure that
the size and total number of t
q
exactly matches the baud rate at which the
network is to run, as the tolerance allowable is normally quoted as +/-1.5%.
The relative settings of TSEG1 and TSEG2 are not so critical as they control
when the hardware samples the data value and there is normally quite a wide
tolerance over which this will work.
If no data other than the baud rate of a network is available a simple ‘rule of
thumb’ is to set the parameters such that there are at least eight time-quanta in
the span of the bit width and that the sample point is 80% through the bit
width.
TABLE 3-1. Typical settings of the CAN Speed Parameters
Baud rate TQUANTA TSEG1 TSEG2
1M 1 5 2
800 K 1 7 2
500 K 2 5 2
250 K 4 5 2
125 K 8 5 2
50 K 16 7 2
20 K 40 7 2
The same three values for these parameters should be used in
every call of the P118 instruction in the datalogger program. NOTE
3.3.4 ID (Parameters 05:, 06:, 07:)
A CAN data frame includes an identifier (ID) which is used by devices on the
network to identify each type of packet on the network. Some standards reserve
certain IDs or ranges of IDs for specific functions. The J1939 SAE standard for
instance reserves certain parts of the ID to identify the type of data, its priority
and its origin (see Appendix C, Application of the SDM-CAN on Networks
Complying with the J1939 SAE Standards, for a discussion of this standard and
use with the SDM-CAN). The SDM-CAN is, however, transparent to any
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